The frequency band of Terahertz (THz, THz=1012 Hz) refers to an electromagnetic radiation range with a frequency from 0.1 THz to 10 THz and a wavelength between microwave and infrared light. Terahertz radiation can provide the capability of ultrafast time resolved spectroscopy because of its short time scale. Terahertz radiation can be used to penetrate through some materials, such as organisms, dielectric medium, and gas phase material. Material's composition, physical state, chemical state, and biological state information can be obtained by analyzing sample materials' transflective terahertz signals. Besides, terahertz radiation will not damage the tested materials because of its broad frequency band and low photon energy, so that the terahertz technology can be used in many fields, such as imaging, spectroscopic analysis, nondestructive testing, and high-speed wireless communication.
Photoconductive sampling and electro-optical sampling are two commonly used terahertz testing technologies, wherein the electro-optical sampling needs low pulse energy and has high sensitivity and a broad probe bandwidth. However, this technology has some defects. The electro-optical sampling is based on electro-optical effect. Terahertz pulse will change the birefringence electro-optical crystal's refractive index when penetrating through the crystal, which causes probe light's polarization state to change. After modulating light path, the illumination difference of two components of the probe light is detected to calculate the intensity of the terahertz pulse. However, this method can only be applied to terahertz signal with low intensity. Applying this method to terahertz signal with high intensity will cause signal distortion, which results in wrong information.